I. Field of the Invention
The present invention relates generally to an optical rear view system and particularly, but not exclusively to an optical system suitable for use as a motor vehicle rear view mirror.
II. Description of the Related Art
As used in this specification the term xe2x80x9cmotor vehiclexe2x80x9d will be understood to refer to any form of vehicle, including watercraft and aircraft as well as motor land vehicles.
Motor vehicle rear view mirrors are essentially required to provide a vehicle operator or user charged with the task of directing the movement of the vehicle, with a readily available source of information concerning the region to the rear of the vehicle. Traditionally motor cars and other road vehicles, as well as aircraft and, to some extent, watercraft, are provided with plane reflectors positioned above and to one side of the driver""s line of sight so that, by displacing his attention momentarily from the view ahead, a driver is able to view the region behind him. A traditional rear view mirror positioned within the passenger compartment provides a driver with a view through the rear screen or back window of the vehicle but this provides a field of view which is too limited, particularly in view of the vehicle""s own rear pillars which obstruct the view, and the main internal rear view mirror is often supplemented with others. Some attempts have been made to expand the field of view by the provision of periscope-type contrivances having sets of reflectors and/or refractors mounted in the vehicle""s roof so that a view can be obtained over the top of the vehicle and therefore unobstructed by the roof-supporting pillars of the vehicle. Such arrangements have not found favour, partly because of the expense of modifying a motor vehicle, and partly because the roof, being an important structural member involved in maintaining the xe2x80x9cstiffnessxe2x80x9d of the vehicle body, considerable additional design effort is required to introduce such an arrangement. Furthermore, the fashion for sunroofs which can slide open and which inevitably involve a component projecting above the roof line, militate against the introduction of such arrangements.
Other attempts to supplement the driver""s rear view include the provision of plane or curved mirrors mounted outside the vehicle, typically on the vehicle""s front doors which can be viewed by the driver through the vehicle side door windows. These additional so-called xe2x80x9cdoorxe2x80x9d mirrors have found almost universal acceptance and are now designed into almost every new vehicle. Door mirrors provide a good rearward vision outside the vehicle to either side thereof making it possible to observe both overtaking and overtaken vehicles. Conventional door mirrors do, however, have certain disadvantages. In order to obtain an appropriate field of view they must project from the maximum exterior body line by as much as 20 cm (often more), and lateral projections of this magnitude represent a considerable physical hazard. Although they are mounted in a releasable manner, the inertia of such elements at high speeds makes them nevertheless a considerable danger. Air resistance due to the increased frontal cross-sectional area of the vehicle also accounts for significant additional fuel consumption and, being positioned right at the furthermost extremities of the lateral extent of the vehicle the driver""s view must be diverted from the straight ahead position through a considerable angle in order to gain a view to the rear door mirror.
The Applicant""s own earlier British Patent No. 2 254 832 and European Patent (UK) No. 0 402 444 describe optical systems which assist in overcoming the disadvantages of conventional rear view door mirrors. In particular, they provide optical systems of low lateral projection which, therefore, do not increase the frontal cross-sectional area of the vehicle to any large extent, and also provide for the view through the rear view mirror to be inboard of the driver""s door and the front passenger door so that the angle through which the driver""s head must be turned in order to gain a view to the rear of the vehicle through the door mirrors is less than it would be with conventional door mirrors.
According to one aspect of the present invention, there is provided an optical rear view system of the type comprising first and second refractors and a reflector in the path of light from an object to an observer, in which the refractors have complementary properties of chromatic dispersion and both refractors are positioned in advance of the reflector in the path of light from an object to an observer.
A major advantage of arranging the components in the above-defined configuration is experienced at low levels of illumination, such as during nighttime driving when the headlights of following vehicles may cause discomfort to the driver of a vehicle in front equipped with conventional rear view mirrors or, more seriously, may even cause loss of vision by dazzling.
In one embodiment of the invention the refractors are prisms oriented with their apices in opposite directions from one another. The reflector may be a plane or curved silvered reflector or a reflector acting by total internal reflection.
Preferably the refractors also have complementary properties as far as chromatic aberrations are concerned, that is the refractor reached second by light travelling from an object to the observer is shaped in such a way and made of such a material as at least partly to compensate for chromatic aberrations introduced by the refractor through which the light first passes. The refractor through which the light first passes will, hereinafter, be referred to as the xe2x80x9cobjectivexe2x80x9d refractor whilst the refractor through which the light passes subsequently will be called the xe2x80x9csecondaryxe2x80x9d refractor.
It is preferred that the objective refractor is made of material having a relatively high refractive index and a relatively low chromatic dispersion in relation to those properties of the secondary refractor whilst this latter has, in relation to the objective refractor, a relatively low refractive index and a relatively high chromatic dispersion. Typically, the objective refractor may be made from an acrylic material whilst the secondary refractor may be made from a polycarbonate, these materials having very favourable relative chromatic properties which dominate the less favourable refractive index relationship. In practice the high chromatic power of polycarbonate permits chromatic aberration to be corrected using a secondary refractor of only half the apex angle of an objective refractor made, for example, of polymethyl methacrylate. The combination of apex angles and refractive indices thus makes the polycarbonate prism behave as if it had a smaller refractive index.
The objective and secondary refractors may be spaced from one another, parallel or inclined as appropriate, and may be formed as a substantially achromatic doublet. Indeed, as far as the geometric location of the secondary refractor is concerned, this may, in one configuration, be positioned at any point between an exit face of the objective refractor and the surface of the reflector. At the latter end of the range of possible positions in this configuration the secondary refractor may be traversed twice by light, that is first by light on its way to the reflector, and then again by light reflected by the reflector towards the observer. In a second configuration the secondary refractor may be positioned in one of a range of positions between the reflector and the observer. The relative inclinations of the refractor faces of the secondary refractor must, of course, be such as to secure appropriate chromatic compensations.
Another particularly important characteristic of the present invention is that the refractors may be formed as triangular prisms or as Fresnel prisms and in either case the angle of inclination between the incident and exit faces of the prisms may vary across the width of the prism. In the case of triangular xe2x80x9cblockxe2x80x9d prisms this variation may be effected incrementally by forming a prism having a plurality of adjacently inclined flat elementary faces, or alternatively may be smoothly and continuously varied in inclination such that the refractors are effectively sections of cylindrical lenses. Indeed, it is not outside the scope of the present invention for the refractors to have varying inclination from top to bottom so that the refractors may be considered as sections of spherical or aspherical lenses which may be either convex, concave, meniscus, plano-convex or plano-concave.
The objective refractor may, furthermore, be made as a double-layer structure of two materials having different refractive index and or chromatic dispersion in order to provide an optical element having the required characteristics.
If the refractors are made as Fresnel prisms it is preferred that the Fresnel facets are in each case formed on only one face and that the said one faces are directed towards one another.
According to a second aspect of the present invention an optical rear view system for a vehicle comprises first and second refractors and a reflector in the path of light from an object to an observer, in which a first or objective refractor has a variation in its xe2x80x9cpowerxe2x80x9d (that is the angle through which a ray of light transmitted through the refractor is diverted) such as to divert light arriving at the system at a small angle to a predetermined longitudinal axis, and bound for the observer, through a greater angle than light arriving at the rear view system bound for the observer at a relatively higher angle to the said axis.
It is also within the ambit of the present invention to provide an optical rear view system as defined hereinabove in which the first or objective refractor has a first end nearer the reflector and a second end further from the reflector, and in which the said second end has opposing, that is incident and exit, faces inclined at a greater angle to one another than the corresponding angle between the same or corresponding faces at the first end of the first or objective refractor.
In a preferred embodiment the second refractor, which may be referred to as a secondary refractor, may be so formed as to divert light by a greater angle at one part than at another and may be so positioned and oriented that the said part which diverts light more greatly receives the more greatly diverted light from the objective refractor.
An optical rear view system formed in accordance with the principles of the present invention may have the said secondary refractor so formed that it varies in diverting power across its width from a first end nearer the reflector, which diverts light most, to a second end further from the reflector which diverts light least.
It is also within the ambit of the present invention to provide an optical rear view system as defined above in which the said second end of the secondary refractor diverts light in an opposite directional sense from the diversion introduced by the first end. In other words the variation in xe2x80x9cpowerxe2x80x9d of the refractor extends to either side of a zero power point at an intermediate position of the refractor.
If the refractors are made as Fresnel prisms adjacent facets may be inclined at angles differing from one another across the width of the prism. Likewise, as mentioned above, the facets may be so shaped as to provide diversion in other planes from that initially discussed, such that the refractors are in the form of Fresnel lenses rather than prisms. Strictly speaking, of course, the refractors will be part-lenses rather than complete lenses and may be cylindrical, spherical of aspherical lenses.
This allows a variation in the field of view to be achieved so that the field of view of the observer is appropriately enlarged or, at least maintained at full size, regardless of the particular configuration of the components in the vehicle.